mx.ipv6.zsttk.net has no MX records.
Question and response
Question
dig mx.ipv6.zsttk.net. MX
Answer
The response has no answer records.
Authority
ipv6.zsttk.net. 86400 SOA ns.zsttk.ru. hostmaster.zsttk.ru. 2018082100 43200 3600 2419200 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0